A transport model of CNTFET is built by solving the Poisson equation and Schrödinger equation within the non-equilibrium Green's function theory. The simulation method can relate the CNTFET transport properties directly with the chiral index of CNT. For the first time, the influences of single HALO and double LDD (HLL) doping structures on the CNTFET are investigated. The results show that under the same gate-source and drain-source voltages, HLL-CNTFET reduces significantly the leakage current and the subthreshold swing and increases on-off current ratio as compared with conventional CNTFET, indicating that this new structure has better gate control ability than conventional CNTFET. HLL-CNTFET possesses a smaller drain-source conductance so that it is more suitable for analog integrated circuits application, and has a smaller threshold voltage shift so shat it can better suppress DIBL effect. The increase of channel electric field strength near the source is beneficial to the increase of the electron transport rate; and the reduction in electric field near the drain is more conductive to the suppression of hot electron effects. This study is helpful for understanding the working mechanism and exploring new features of CNTFET.